Random access memories which must retain vital data, even after being subjected to nuclear radiation while writing or restoring data, can be rendered immune to the radiation effects by storing small quantities of data doubly, triply or multiply redundantly in the memory. This enables a single word loss to be remedied by reference to other error-free copies of the affected word. However, practical limitations on memory size prohibit the entire contents of a typical memory from being stored in a redundant manner.
Memory word loss may be obviated in a random access memory without the need for redundant storage, in the case of the fixed data, by the use of the plated wire type non-destructive readout memory. If suitable circumvent circuitry is provided to protect the words stored in the plated wire memory, the memory will be safe from the effects of nuclear radiation insofar as the fixed data is concerned. This is because of the non-destructive readout characteristics of this type of memory. The radiation hardened plated wire memory, therefore, may be used for the storage of fixed data since under normal operation the fixed data will be read only from the memory and, due to the non-destructive readout properties of the memory, any word being read during a nuclear event can be reconstituted from the memory itself. However, the radiation hardened plated wire memory does present problems when used for variable data storage, since it is vulnerable insofar as a word being written into memory is concerned.
Thus, the radiation hardened plated wire memory is not immune to the effects of nuclear radiation which occurs when a particular variable data word is being updated, since such a particular data word being written into the plated wire memory during a nuclear event could be lost. Therefore, without further protection, the plated wire memory is not suitable for the storage of variable data.
Moreover, the non-destructive readout plated wire memory is more costly than the destructive readout magnetic core type, and significant cost problems arise when the non-destructive readout type of memory is used for the fixed data storage. The option of using the less expensive destructive readout memory for fixed data storage presents a problem, however, since it is susceptible to the effects of nuclear radiation on both the words being written into the memory or read from the memory. This is because the destructive readout characteristics of the magnetic core memory create the need for restoring data after each readout operation. This makes this type of memory vulnerable to the radiation effects when data is being read from the memory or written into the memory. Redundant storage of all the contents of a destructive readout magnetic core memory is equally impractical.
The invention disclosed in U.S. Pat. No. 4,031,374, assigned to the present assignee, provides an improved system for rendering the destructive readout magnetic core random access memory immune to the effects of nuclear radiation.
The protective system of the patent includes circumvent circuitry which responds to the detection of a nuclear radiation event to isolate magnetic core random access memory from the effects thereof. The circumvent circuitry enables unaddressed memory locations to be protected. However, it is extremely difficult by circumvent circuitry to control the required circuits in the memory should exposure to the nuclear radiation occur during an actual write or read/restore operation. This means that the word being read from the core memory, or written into the memory, during the exposure may be lost. As described, the plated wire memory operates on a non-destructive readout basis which safeguards a stored word from being destroyed during a read cycle, even if it is being read when a nuclear event occurs. In the case of the magnetic core memory, however, the word desired in each read operation must be restored in a subsequent write operation, so that the affected word may be lost during the presence of nuclear radiation. Additional means are provided to reconstruct words being accessed during the nuclear event. In the case of the plated wire memory, the additional means is required only to reconstruct a word actually being written into the memory during the nuclear event. In the case of the core memory, however, the corrective measures must be taken with respect to words being read or written during the nuclear event.
The system disclosed in the patent discloses apparatus and procedures for taking such corrective measures. Although the system of the patent operates generally satisfactorily where a single nuclear event occurs or where there is a relatively long time period between nuclear events, it is not capable of taking corrective steps where relatively closely spaced nuclear upsets occur which are capable of upsetting the electronics before full correction may be obtained.
The corrective measures disclosed in the patent render the memory system immune to the effects of nuclear radiation, and the like. The system includes circumvent circuitry which serves to limit all currents in the memory access network to safe values during a radiation event, so as to limit data loss to a maximum of one word per event and to prevent burnout of the memory and of its associated components. The system also serves to correct single word errors which can occur to words being written or read during the radiation pulse. The single word error correction is achieved by partitioning the data words in the memory into blocks; by providing an error correction word for each such block, and by reconstructing from the error correction word the word being read or written during the radiation event. This may be achieved by first loading all zeros into the memory address of the affected word, and then reconstructing the affected word by the provision of an "Exclusive-OR" logic means which forms a computed error correction word from the entire block, including the affected word (which is now zeros), and the original error correction word. The computed error correction word is a reconstruction of the affected data word.